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      Circular RNAs Are the Predominant Transcript Isoform from Hundreds of Human Genes in Diverse Cell Types

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          Abstract

          Most human pre-mRNAs are spliced into linear molecules that retain the exon order defined by the genomic sequence. By deep sequencing of RNA from a variety of normal and malignant human cells, we found RNA transcripts from many human genes in which the exons were arranged in a non-canonical order. Statistical estimates and biochemical assays provided strong evidence that a substantial fraction of the spliced transcripts from hundreds of genes are circular RNAs. Our results suggest that a non-canonical mode of RNA splicing, resulting in a circular RNA isoform, is a general feature of the gene expression program in human cells.

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          Most cited references20

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          A coding-independent function of gene and pseudogene mRNAs regulates tumour biology

          The canonical role of messenger RNA (mRNA) is to deliver protein-coding information to sites of protein synthesis. However, given that microRNAs bind to RNAs, we hypothesized that RNAs possess a biological role in cancer cells that relies upon their ability to compete for microRNA binding and is independent of their protein-coding function. As a paradigm for the protein-coding-independent role of RNAs, we describe the functional relationship between the mRNAs produced by the PTEN tumour suppressor gene and its pseudogene (PTENP1) and the critical consequences of this interaction. We find that PTENP1 is biologically active as determined by its ability to regulate cellular levels of PTEN, and that it can exert a growth-suppressive role. We also show that PTENP1 locus is selectively lost in human cancer. We extend our analysis to other cancer-related genes that possess pseudogenes, such as oncogenic KRAS. Further, we demonstrate that the transcripts of protein coding genes such as PTEN are also biologically active. Together, these findings attribute a novel biological role to expressed pseudogenes, as they can regulate coding gene expression, and reveal a non-coding function for mRNAs.
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            Genomewide characterization of non-polyadenylated RNAs

            Background RNAs can be physically classified into poly(A)+ or poly(A)- transcripts according to the presence or absence of a poly(A) tail at their 3' ends. Current deep sequencing approaches largely depend on the enrichment of transcripts with a poly(A) tail, and therefore offer little insight into the nature and expression of transcripts that lack poly(A) tails. Results We have used deep sequencing to explore the repertoire of both poly(A)+ and poly(A)- RNAs from HeLa cells and H9 human embryonic stem cells (hESCs). Using stringent criteria, we found that while the majority of transcripts are poly(A)+, a significant portion of transcripts are either poly(A)- or bimorphic, being found in both the poly(A)+ and poly(A)- populations. Further analyses revealed that many mRNAs may not contain classical long poly(A) tails and such messages are overrepresented in specific functional categories. In addition, we surprisingly found that a few excised introns accumulate in cells and thus constitute a new class of non-polyadenylated long non-coding RNAs. Finally, we have identified a specific subset of poly(A)- histone mRNAs, including two histone H1 variants, that are expressed in undifferentiated hESCs and are rapidly diminished upon differentiation; further, these same histone genes are induced upon reprogramming of fibroblasts to induced pluripotent stem cells. Conclusions We offer a rich source of data that allows a deeper exploration of the poly(A)- landscape of the eukaryotic transcriptome. The approach we present here also applies to the analysis of the poly(A)- transcriptomes of other organisms.
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              Splicing with inverted order of exons occurs proximal to large introns.

              Following our studies which showed that the alpha and beta exons of the chicken c-ets-1 gene are not conserved in the human homologue, we succeeded in identifying a novel human c-ets-1 transcript in which the normal order of exons is scrambled. By PCR and RNase protection assays, we demonstrated that while the order of exons is different from that in genomic DNA, splicing of these exons in aberrant order occurs in pairs and at the same conserved consensus splice sites used in the normally spliced transcript. The scrambled transcript is non-polyadenylated and is expressed at much lower levels than the normal transcript. It is not the consequence of genomic rearrangement at the ets-1 locus nor is it due to the transcription of any ets-1 pseudogene. These results confirm previous observations of scrambled splicing.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2012
                1 February 2012
                03 February 2012
                : 7
                : 2
                : e30733
                Affiliations
                [1 ]Department of Biochemistry, Stanford University School of Medicine, Stanford, California, United States of America
                [2 ]Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, California, United States of America
                [3 ]Department of Pediatric Hematology/Oncology, Stanford University School of Medicine, Stanford, California, United States of America
                The John Curtin School of Medical Research, Australia
                Author notes

                Conceived and designed the experiments: JS CG PW PB. Performed the experiments: CG JS PW. Analyzed the data: JS CG PW PB. Wrote the paper: JS CG PB. Designed the analysis of the RNA-Seq data: JS PB. Obtained permission for the use of and access to clinical samples: NL.

                Article
                PONE-D-11-22239
                10.1371/journal.pone.0030733
                3270023
                22319583
                4bd45ef3-43c6-4d6a-8156-4b9c645ca926
                Salzman et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
                History
                : 7 November 2011
                : 28 December 2011
                Page count
                Pages: 12
                Categories
                Research Article
                Biology
                Computational Biology
                Genomics
                Genome Analysis Tools
                Genetics
                Gene Expression
                Genomics
                Genome Analysis Tools

                Uncategorized
                Uncategorized

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